Andy, I take you meant 8R8 as filament resistor?
This config is worth trying to test if the series resistor has a positive impact in the sound!
For Rod's filament supply, send him a message. The schematic isn't public domain, but the boards are very cheap and he will give you all the details.
For Lundahl transformers, probably the easiest is to send a message to Thomas Mayer - see also his website. You are not too far away from him.
VinylSavor: New Lundahl Line Output / Interstage Transformer
Which Lundahl transformer you need will depend on the ratio of stepdown you like, as he explains.
For Lundahl transformers, probably the easiest is to send a message to Thomas Mayer - see also his website. You are not too far away from him.
VinylSavor: New Lundahl Line Output / Interstage Transformer
Which Lundahl transformer you need will depend on the ratio of stepdown you like, as he explains.
I don't know if anyone has details to hand of the posts that contain schematics - it's a bit of a trek up the North Face of this thread... I find it easier to message/email people concerned. Ale has all sorts of useful stuff on his site, but like Thomas's site the blog is turning into a very long affair also!!
Ale Moglia | Bartola Valves
Ale Moglia | Bartola Valves
I just got through reading through all the posts in this thread the last few days. Maybe it would be a good idea to have a post that indexes most of the posted schematics:
Jim de Kort's site
Kevin Kennedy's PF article
Keven Kennedy's Schematic PDF
Where to begin? At the beginning of this thread
Coolzero 1
revintage 1
revintage 2
mach1 sneaks in a 12B4A schematic
Richard's double-fisted 156C choke load
coolzero's hum avoidance diagram
coolzero 2
revintage 3
andyjevans's 10Y preamp
regal's 26 driving 71A
Rod Coleman's DHT linestage
Rod Coleman's DHT linestage with power supply
andyjevans 26 with AZ1 rectifier
coolzero 4
mogliaa's 26 testbed
Original Burnedfingers
euro21
mogliaa's 01A
mogliaa's 26 source follower
dhtrob's 26
andyjevans still can't find the rotate 90° command
Rod Coleman's 26 with a filament-on-filament bias with IDHT output
euro21
mogliaa version 2
Nightpuma's 26
dhtrob 2 (now with grilled pencils)
H.Michi
andyjevans 26 with VR tubes.
andyjevans 26 with VR tubes and CCS
Jim de Kort's site
Kevin Kennedy's PF article
Keven Kennedy's Schematic PDF
Where to begin? At the beginning of this thread
Coolzero 1
revintage 1
revintage 2
mach1 sneaks in a 12B4A schematic
Richard's double-fisted 156C choke load
coolzero's hum avoidance diagram
coolzero 2
revintage 3
andyjevans's 10Y preamp
regal's 26 driving 71A
Rod Coleman's DHT linestage
Rod Coleman's DHT linestage with power supply
andyjevans 26 with AZ1 rectifier
coolzero 4
mogliaa's 26 testbed
Original Burnedfingers
euro21
mogliaa's 01A
mogliaa's 26 source follower
dhtrob's 26
andyjevans still can't find the rotate 90° command
Rod Coleman's 26 with a filament-on-filament bias with IDHT output
euro21
mogliaa version 2
Nightpuma's 26
dhtrob 2 (now with grilled pencils)
H.Michi
andyjevans 26 with VR tubes.
andyjevans 26 with VR tubes and CCS
Last edited:
I just completed modification to my 26 pre-amp "from battery to the fillament biasing".
As a result I feel the sound surely improved.
However, I feel a residual hum? noise (0.31mV) which was not included in battery bias. I measured by ac voltmeter(Kenwood VT-171) at the output terminal (600 ohm).
Under such circumstances, I wonder are there any suggestions or idea in getting rid of the said residual noise?
As a result I feel the sound surely improved.
However, I feel a residual hum? noise (0.31mV) which was not included in battery bias. I measured by ac voltmeter(Kenwood VT-171) at the output terminal (600 ohm).
Under such circumstances, I wonder are there any suggestions or idea in getting rid of the said residual noise?
Attachments
Hiro, such a high level of noise is not expected - especially with the excellent filtering of raw dc shown in your circuit.
my suggestions for checks:
1. Please measure ac voltage (common-mode) at input of filament Regulator: Meter + at Raw dc +, Meter - connected at cathode of 26 (GND).
2. Please measure ac voltage (differential-mode) at input of filament Regulator: Meter + at Raw dc +, Meter - at Raw dc -. This should be <100mV
3. Please measure ac voltage (common-mode) at output of filament Regulator: Meter + at filament +, Meter - connected at cathode of 26 (GND). This should be < 1mV
differential mode ac voltage (Filament + to Filament -) should be << 1mV
4. filament transformer has low capacitance from primary to secondary? 50 to 90pF is a good value.
Also, transformer construction is E-I ?
4. possibly, a capacitor of 1000uF to 3300uF connected at Regulator PCB (input terminals), may be useful. Low impedance type: Panasonic FC, Nichicon PM etc.
5. Snubber network R-C (R=47R 1/2W; C=47nF stacked film) placed across filament transformer secondary is helpful to eliminate leakage-inductance waveform problems (large spikes).
my suggestions for checks:
1. Please measure ac voltage (common-mode) at input of filament Regulator: Meter + at Raw dc +, Meter - connected at cathode of 26 (GND).
2. Please measure ac voltage (differential-mode) at input of filament Regulator: Meter + at Raw dc +, Meter - at Raw dc -. This should be <100mV
3. Please measure ac voltage (common-mode) at output of filament Regulator: Meter + at filament +, Meter - connected at cathode of 26 (GND). This should be < 1mV
differential mode ac voltage (Filament + to Filament -) should be << 1mV
4. filament transformer has low capacitance from primary to secondary? 50 to 90pF is a good value.
Also, transformer construction is E-I ?
4. possibly, a capacitor of 1000uF to 3300uF connected at Regulator PCB (input terminals), may be useful. Low impedance type: Panasonic FC, Nichicon PM etc.
5. Snubber network R-C (R=47R 1/2W; C=47nF stacked film) placed across filament transformer secondary is helpful to eliminate leakage-inductance waveform problems (large spikes).
Hi Rod, Thank you very much for your very quick responce!!
I will check following your advices.
BTW, my previous schematic was wrong one.
Attachments
Hiro,
What kind of Line Output Transformer do you use within you preamp? Seems interesting that it has finemet core. Is that feastrex? How is the sound?
Ty094,
I bought a custom-designed OPT (FM15K(200H)-150x2) via my friend and honestly I do not know the maker`s name.
As far as I know, they are small company in Saitama and they may be supply the finemet core OPT`s to Andix Audio or may be Noguchi.
Specification of FM15K(200H)-150x2:
winding ratio: 1:0.22
Inductance: 212H(5Hz) 15K
216H(100Hz)
256H(1KHz)
Maximum current: 15mA/15K
DCR primary 783 ohm x2
Secondary 43ohm x2
Frequency response 8Hz ~ 30KHz (-3dB)
The sound impression is warm, brilliant and less distortion.
Hi Michi,
Do you compare the sound between your pre 71A and 26 ? Which makes you satisfied ?
Regards,
QA.